3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs.
The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In LTE, the eNodeB manages the radio resources in the cells, and is directly connected to a Core Network (CN), as well as to neighboring eNodeBs via an X2 interface.
During Release 12, the LTE standard has been extended with support of device to device (D2D) (specified as “sidelink”) features targeting both commercial and Public Safety applications. Some applications enabled by Rel-12 LTE are device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.
D2D communications may be extended to support Vehicle-to-X (V2X) communications, which includes any combination of direct communication between vehicles, pedestrian carried devices, and infrastructure mounted devices. V2x communication may take advantage of available network (NW) infrastructure, although at least basic V2x connectivity can be possible in case of lack of available network infrastructure. Providing an LTE-based V2x interface may be economically advantageous because of the LTE economies of scale and it may enable tighter integration between communications with the NW infrastructure (vehicle-to-infrastructure (V2I)), (vehicle-to-pedestrian (V2P)), and (vehicle-to-vehicle (V2V)) communications, as compared to using a dedicated V2x technology. The term sidelink is used in the 3GPP specifications to refer to the transmission of messages directly between UEs; that is, without passing through an eNodeB. Sidelink is used for realizing D2D communications, V2x and x2V communications, ProSe (Proximity Services), etc. In LTE, sidelink communications take place over the PC5 interface, whereas cellular communications (i.e., uplink and downlink) take place over the Uu interface. Although the messages are exchanged directly between UEs, communication may or may not be controlled by an eNodeB. For example, the eNodeB may set pools of time-frequency resources for sidelink communications, or it may schedule the sidelink communications in specific time-frequency resources. FIG. 1 is a schematic diagram illustrating V2X scenarios for an LTE-based Radio Access Network (NW). As shown in FIG. 1, V2I (Vehicle to Infrastructure) communications may be provided between a vehicle and the radio access network (RAN), V2V (Vehicle to Vehicle) communications may be provided directly between different vehicles (without communicating through the radio access network), and V2P (Vehicle to Pedestrian) communications may be provided directly between a vehicle and a device held by the person or pedestrian (e.g., a smartphone, a tablet computer, etc.). V2X communications are meant to include any or all of V2I, V2P, and V2V communications.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc.
The European Telecommunications Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
A CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. The CAM message also serves as active assistance to safety driving for normal traffic. Devices check availability of a CAM message every 100 ms, yielding a maximum detection latency requirement is not more than 100 ms for most CAM messages. However, the latency requirement for Pre-crash sensing warning is not more than 50 ms.
A DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100 ms, and the requirement of maximum latency is not more than 100 ms.
The package size of CAM and DENM message can vary from more than 100 to more than 800 bytes, although the typical size is around 300 bytes depending on the specific V2X use case, message type (e.g. DENM can be larger than CAM), and depending on the security format included in the packet (e.g., full certificate or certificate digest). The message is supposed to be detected by all vehicles in proximity.
The Society of the Automotive Engineers (SAE) has defined a Basic Safety Message (BSM) for Dedicated Short-Range Communications (DSRC) with various defined messages sizes. Based on the importance and urgency of the messages, the BSMs are further classified into different priorities. DSRC are one-way or two-way short-range to medium-range wireless communication channels specifically designed for automotive use and a corresponding set of protocols and standards.
Radio Resource Booking
In V2x communications, two major types of traffic are distinguished: recurrent traffic and event-triggered traffic. For recurrent traffic, the transmitted packets arrive regularly (e.g., they may be strictly periodic or have some deviation from an average periodicity). One efficient way to schedule recurrent-traffic V2x transmissions is to use radio resource booking. In resource allocation using resource booking a UE can book radio resources in advance for transmitting the next packet (including all the retransmissions). The minimum time span of a booking is usually taken to be the minimum time between two consecutive packets (e.g., the minimum message periodicity). Similarly, the maximum time span of a booking is usually taken to be the maximum time between two consecutive packets (e.g., the maximum message periodicity). For example, in V2X the time interval between the generation of two consecutive CAM messages may not be lower than 100 ms (in the absence of congestion control) and may not exceed 1 s. Thus, it is reasonable to allow bookings for 100 ms, 200 ms, . . . , or 1 s, as it is currently being considered by 3GPP. Usually, the UE signals the booking information to other UEs. This allows a receiving UE to predict the future utilization of the radio resources by reading received booking messages and schedule its current transmission to avoid using the same resources. To do so, a UE needs to sense the channel for some time duration preceding the (re)selection trigger to gather booking messages. In addition, it may also be possible to transmit unbooking messages that release previously booked resources.